The invention relates to a discharge lamp of the type having a base, an outer envelope, an arc tube with a pair of opposed pinches and a metal halide filling, a space between the arc tube and the envelope, a frame supporting the arc tube with straps around the pinches, and a getter strip fixed relative to at least one of the straps.
The function of the getter strip is to absorb impurities in the inert gas or nitrogen which fills the space between the arc tube and the outer envelope. Zirconium alloys such as ST-101 (Zr84Al16) are widely used in metal halide lamps as a getter to absorb impurities such as hydrogen, water vapor, and hydrocarbons. These impurities are detrimental to lamp performance. For example hydrogen can diffuse through the quartz envelope of an arc tube and form hydrogen iodide, which causes voltage spikes as the lamp warms up, thereby causing the dropback or RMS voltage to increase. In the worst case, excessive hydrogen iodide in the system would cause the lamp to extinguish.
The best results are achieved when the getter temperature is within an optimal range, which dictates that the getter be mounted next to the arc tube. The ST-101 getter in high wattage metal halide lamps is typically welded to the arc tube straps at an angle of less than 15xc2x0, for example 5xc2x0 or even parallel to the strap.
However the inventor has discovered that this orientation renders the getter somewhat non-functional when the lamp is operated horizontally.
FIGS. 1 and 2 show the convection flow patterns in a metal halide lamp having an ellipsoidal outer envelope with an arc tube mounted on its major axis. As shown in FIG. 1, convection flows are generated in the lamp during vertical operation as shown by the ovals. The inert gas in the space between the arc tube and the outer envelope flows upward adjacent to the hot surface of the arc tube, then back down toward the base adjacent to the cooler surface of the envelope. During horizontal operation, as shown in FIG. 2, convection flows are generated as indicated by the modified ovals. The inert gas flows upward around the circumference of the arc tube, then back down around the ends of the arc tube and the cooler surfaces of the outer envelope. Flows from opposite ends converge at the minor axis of the ellipsoid.
During vertical operation, as shown in FIG. 1, the getter is positioned transverse to the flow and therefore is well positioned to absorb impurities as they flow by. However during horizontal operation, as shown in FIG. 2, the getter may not be properly exposed to the flow and therefore may not absorb impurities efficiently. This can result in poor performance over life such as voltage rise and poor lumen maintenance. The lamps may even cycle on and off.
FIGS. 3 and 4 show experimental data for a metal halide type MH 1000/U lamp having getters oriented at less than 15xc2x0, actually about 5xc2x0, with respect to the strap. FIG. 3 shows voltage spikes due to the presence of hydrogen in the arc tube (hydrogen spikes) during continuous operation with the lamp in three operating positions, which are base-up, base-down, and horizontal. With the lamp operated horizontally, the spikes reach a maximum of 250 volts at 275 hours. In both vertical positions the spikes are negligible. FIG. 4 shows the voltage from the time of an arc until it drops back (dropback voltage). This rises steadily to a maximum of 45 volts at 275 hours in horizontal operation, but stabilizes at about 23 volts during vertical operation.
U.S. application Ser. No. 09/176,550 discloses a metal halide lamp of the type to which the invention relates, having an ellipsoidal outer envelope and ST 101 getters welded to the pinch straps at an angle of about 15xc2x0 to the plane of the pinch straps.
EP 0497225 discloses a high pressure sodium lamp having a BaAl ring arranged in a vacuum space between the ceramic arc tube and the outer envelope, at an angle between 30 and 45xc2x0 to the geometric axis of the lamp. During manufacture of the lamp, the ring is heated by induction heating so that barium evaporates and deposits layers on the stem and on the inside surface of the envelope. During operation of the lamp, the layer on the stem reaches temperatures of 350-420xc2x0 C., which is suitable for binding oxygen in the space. The layer on the envelope reaches temperatures of 120-250xc2x0 C., which is suitable for absorbing hydrogen. The ring itself has no function during operation. There are virtually no convection currents in the space because there is no fill gas; the oxygen and hydrogen are only contaminants in a vacuum space.
GB 1,333,272 discloses a ZrAl getter ring mounted in the end of a 400 watt T-bulb type lamp remote from the arc tube. No hydrogen gettering problems have been observed in lamps of this type.
JP 57-84557 discloses a high pressure sodium lamp having zirconium getters mounted to the support frame for the ceramic arc tube, and a rare gas such as xenon in the space. The arc tube is cylindrical because ceramic cannot be formed with pinches, and there is no indication that the getter strips are mounted at any particular orientation.
It is an object of the present invention to provide a metal halide discharge lamp with good gettering of hydrogen and other impurities in the space between the arc tube and the outer envelope, regardless of operating position of the lamp.
According to the invention, a getter strip is fixed adjacent to one of the pinches closing the arc tube, at an angle of at least 30xc2x0 to the pinch plane. When the lamp is operated in a horizontal position, the getter strip is thus positioned transversely to convection flow around the pinch, regardless of angular orientation.
Preferably, getter strips are fixed adjacent to both pinches on opposite sides of the pinch plane, at angles of at least 30xc2x0 to the pinch plane.
Preferably, the getter strips are a zirconium aluminum alloy, in particular ST 101, and are welded directly to the straps which hold the pinches of the arc tube. With the strips extending from the straps at 45xc2x0 in a 1000 watt lamp having an ellipsoidal envelope and a nitrogen fill in the space, hydrogen spikes were, virtually eliminated when the lamp was operated in a horizontal position, and dropback voltage values were comparable to those in other operating positions.
The test results are interpreted to mean that the orientation of the getter strip at angles over 30xc2x0 to the pinch plane results in better absorption of impurities than orientation at more acute angles.